Current detection circuit and switching power supply

1. A current detection circuit for detecting the current flowing in a switching device, comprising: an auxiliary switching device being provided in parallel with said switching device and being ON at least when said switching device is ON, an offset voltage source for generating an offset voltage, and a compensation circuit for adjusting the output current of said auxiliary switching device so that the potential obtained by adding said offset voltage to the output potential of said switching device becomes equal to the output potential of said auxiliary switching device, wherein said offset voltage source comprises an offset resistor device, and a current source circuit for passing a current in said offset resistor device, and said offset resistor device is an FET having the same channel type as that of said switching device.

2. The current detection circuit in accordance with claim 1 , wherein said switching device and said auxiliary switching device are FETs having the same channel type.

3. The current detection circuit in accordance with claim 1 , wherein said compensation circuit comprises: a differential amplifier to which the potential obtained by adding said offset voltage to the output potential of said switching device and the output potential of said auxiliary switching device are input, and a variable impedance device, connected to the output terminal of said auxiliary switching device, the impedance of which changes depending on the output of said differential amplifier.

4. A switching power supply comprising a switching device, an inductor connected to said switching device, a rectifier circuit for rectifying the output of said inductor, smoothing means for smoothing the current flowing in said rectifier circuit, a current detection circuit for detecting the current of said switching device, and a control circuit, to which the current detection signal from said current detection circuit and the output voltage output from said smoothing means are input, for turning ON/OFF said switching device to control said output voltage, wherein said current detection circuit comprises: an auxiliary switching device being provided in parallel with said switching device and being ON at least when said switching device is ON, an offset voltage source for generating an offset voltage, and a compensation circuit for adjusting the output current of said auxiliary switching device so that the potential obtained by adding said offset voltage to the output potential of said switching device becomes equal to the output potential of said auxiliary switching device, wherein said offset voltage source comprises an offset resistor device, and a current source circuit for passing a current in said offset resistor device, and said offset resistor device is an FET having the same channel type as that of said switching device.

5. The switching power supply in accordance with claim 4 , wherein said control circuit generates a control signal for controlling the output voltage and turns ON/OFF said switching device so that the current detection signal from said current detection circuit follows said control signal.

6. The switching power supply in accordance with claim 5 , wherein said rectifier circuit has a rectifier switching device, and said control circuit turns ON/OFF said switching device and said rectifier switching device alternately.